TISSUES OF THE BODY. 



161 



vera containing the vessels of the dermis, with the connective tissues 

 of mucous membrane and true glands, take their rise from this source. 



It is evident that the changes must be very great in the middle layer 

 which produce from it such structures. 



To many of these changes we shall have to refer again in subsequent 

 pages, but for the present our attention must be principally directed to 

 the formation of numerous cavities in the middle plate appearing in the 

 course of its development. Thus it is that the serous sacs originate, as also 

 the bursae and sheaths of tendons. Thus is formed the most intricate 

 of all systems of canals, namely, that of the blood and lymphatics. 

 Together with this formation of cavities, also, we must expect besides 

 to find a whole series of epithelial coatings springing up. 



The latter have much about them that is peculiar. If we except the 

 more circumscribed laminations as they appear in synovial membranes, 

 they almost always consist of a single layer of thin flat scales ( 87) with- 

 out the transient nature of the two other forms of epithelium. Further, as 

 observation of the circulatory system teaches, such an epithelial tunic 

 may acquire sufficient strength by cohesion of its cells, as to fit it to form 

 the chief part of the finer and more minute canals of the former. But 

 these epithelia of the intermediate germinal plate do not possess the power 

 of yielding in continuous transition the secreting cells of glands, nor are 

 they able to develope any physiological function similar to glandular 

 activity. On the other hand, they are remarkable for the great ease with 

 which the fluids of the blood transude through them, which is far from 



Fig. 152. Skin, withru<1iments of a hair, in 

 a human embryo of sixteen weeks old. a, 

 the superficial layers of cells of the epi- 

 dermis; b, the deeper; m m, cells of the 

 rudimentary hair; ', transparent mem- 

 brane clothing them. 



itf. 153 Epidermis from the neighbourhood of 

 the head of a sheep-embryo of 4 in. in length. 



1, epidermal cells, of the most STtperficial layer ; 



2, from deeper lamina; 3, vertical section of 

 the same; 4, cuticle from the free edge of the 

 eyelid. 



being the case with the epithelial structures of the corneous layer. If we 

 are asked for another contrast, it is to be found in the nonvascularity 

 of the subjacent tissue, of this last species of epithelium compared to 

 the great vascular! ty of the other two kinds. 



This intermediate species has been termed false epithelium, or endo- 

 thelium, by His. 



As regards the epithelium of the corneous layer, Koellilter found the 

 epidermis in the human embryo of five weeks old to consist already 

 of two laminae of nucleated cells : a superficial, made up of delicately- 

 edged polyhedral elements of 0'0275-0'0451 mm., with round nuclei, 

 measuring 0'0090-0'0136 mm.; and a deeper layer, in which the cells 



